network: fix private sandbox netns #917

Job	Run time
tests (vmm/sandbox, --all-features)	3m 4s
checks (vmm/sandbox, --all-features)	1m 41s
tests (vmm/task, --all-features)	2m 50s
checks (vmm/task, --all-features)	1m 59s
tests (shim, --all-features)	2m 33s
checks (shim, --all-features)	1m 57s
tests (quark, --all-features)	2m 27s
checks (quark, --all-features)	1m 58s
tests (runc, --all-features)	2m 28s
checks (runc, --all-features)	1m 58s
tests (wasm, --features=wasmedge, 0.11.2)	2m 46s
checks (wasm, --features=wasmedge, 0.11.2)	1m 58s
tests (wasm, --features=wasmtime)	3m 18s
checks (wasm, --features=wasmtime)	1m 58s
	32m 55s

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